1. Field of the Invention
The present invention relates to an X-ray mask structure used in X-ray lithography, and an X-ray exposure process using the same.
2. Related Background Art
The X-ray mask structure used in X-ray lithography is mainly comprised of an X-ray transmissive membrane, an X-ray absorber held on the X-ray transmissive membrane, and a holding frame that holds the X-ray transmissive membrane.
In the field of X-ray lithography, a high resolution of a half-micron order has been hitherto required for the pattern line width.
The main reason to decrease the high resolution originates from the mask structure, and is based on the following problems involved in the X-ray mask structure itself:
1) A problem of deterioration of an X-ray transmission performance originating from an X-ray transmissive membrane; PA1 2) A problem of the strain produced in an X-ray transmissive membrane, caused by internal stress; PA1 3) A problem of the strain produced in an X-ray absorber, caused by internal stress; and PA1 4) A problem of the adhesion between an X-ray transmissive membrane and an X-ray absorber. PA1 said X-ray transmissive membrane comprises a layer of aluminum nitride; and PA1 said X-ray absorber comprises a nitride of heavy metal. PA1 said X-ray transmissive membrane comprises a layer of aluminum nitride; PA1 said X-ray absorber comprises a nitride of heavy metal; and PA1 an intermediate layer mainly composed of aluminum, a heavy metal and nitrogen is held between said X-ray transmissive membrane and said X-ray absorber. PA1 said X-ray transmissive membrane comprises a layer of aluminum and; PA1 said X-ray absorber comprises a nitride of heavy metal. PA1 said X-ray transmissive membrane comprises a layer of aluminum nitride; PA1 said X-ray absorber comprises a nitride of heavy metal; and PA1 an intermediate layer mainly composed of aluminum, a heavy metal and nitrogen is held between said X-ray transmissive membrane and said X-ray absorber.
In order to solve the above problems, various technical improvements have been hitherto made. For example, U.S. Pat. No. 4,677,042 discloses that an X-ray mask structure in which aluminum nitride is used in the X-ray transmissive membrane, and a heavy metal such as tungsten or tantalum is used as the X-ray absorber, can solve, in particular, the above problem 1). Japanese Patent Laid-Open Nos. 63-76325 and 63-32425 also disclose that an X-ray mask structure in which silicon nitride, silicon oxide or boron nitride is used in the X-ray transmissive membrane, and a nitride of tungsten, a nitride of tantalum or a nitride of titanium-tungsten is used as the X-ray absorber, can solve, in particular, the above problems 3) and 4).
In practical use in X-ray lithography, however, even the X-ray mask structures disclosed in the above prior arts have caused the following new problems.
Namely, in the practical use in X-ray lithography;
(i) it is conventional to carry out pattern exposure a thousand times or as many as several ten thousand times using the same X-ray mask structure, and as a result of such repeated pattern exposure, a strain is produced with time in the X-ray mask structure, resulting in a decrease in the resolution of patterns; and
(ii) the X-ray mask structure is often used under a reduced pressure for the purpose of preventing attenuation of X-rays at the time of exposure, where a strain is also produced in the X-ray mask structure as a result of repeated pressure variations that occur when an exposing object (e.g., a wafer) is attached to or detached from a device, resulting in a continual decrease with time of the resolution of patterns. Thus, the conventional X-ray mask structures have been unsatisfactory in respect of the durability thereof.